Field
The embodiments relate generally to the field of recycling, and more particularly but not exclusively, to continuous conversion of waste plastics into liquid fuels.
Discussion of Related Field
Increased global consumption of non-renewable materials, such as, crude oil, coal and natural gas is leading to a rapid depletion of their finite natural reserves. Today, the need, therefore for developing economically feasible and scalable technologies that can recycle products which are manufactured using such non-renewable material cannot be over-emphasized.
Plastics are polymers manufactured from feedstock derived from non-renewable crude oil. The manufacturing and consumption of plastics has increased substantially over the years. However, once consumed, the plastic material is not easy to dispose, as it does not biodegrade naturally. Consequently, plastic waste is a large strain on existing disposal methods, which include landfill and incineration. Landfill space is becoming scarce and expensive, a problem exacerbated by the fact that plastic waste is more voluminous than other waste type. On the other hand, incineration of plastics to recover energy, produces toxic gaseous products, which only shifts a solid waste problem to an air pollution one.
Mechanical recycling remains the dominant method to recycle plastic wastes. However, such recycling is limited by the compatibility between different types of polymer resins. Presence of a polymer dispersed in a matrix of a second polymer may dramatically change the properties and hinder the possibilities to use it in the conventional applications. A good example of this is the impacts of polyvinyl chloride (PVC) during polyethylene terephthalate (PET) processing. A small amount of PVC in the recycled PET strongly reduces the commercial value of the latter. Another problem with mechanical recycling is the presence of different colors in plastic waste of products made of the same resin, which usually impart an undesirable grey color to the recycled plastic.
Current methods for management of plastic wastes face a host of challenges. Technological solutions, which accepts commingled different resin plastics often lack commercial viability for scaling and replication. Further, the high cost of recycling plastics as per a specific resin code (e.g. separation of HDPE from mixed plastic waste) and the low cost of virgin plastic with which recycled plastic has to compete also poses challenge to recycling of plastic wastes.
Owing to issues with mechanical recycling, land filling and incineration of waste plastics, the emphasis has increased recently for development of pyrolytic methods which use heat (thermal) or heat and catalyst (thermocatalytic) in an inert atmosphere for degradation of waste plastics to obtain liquid fuels. During pyrolysis, the polymeric materials are heated to high temperatures, such that the macromolecular hydrocarbon chains contained in them are scissioned (cracked) and broken into smaller hydrocarbon chains. This results in formation of gaseous and liquid products which have a wide distribution of carbon number. The major pyrolytic products coming from such a process can be divided into:                i. A hydrocarbon liquid fraction, which comprises of paraffins, iso-paraffins, olefins, napthenes and aromatics and has a gross calorific value between 18900-20700 BTU/lb        ii. A gas fraction, which comprises mostly of C1-C4 hydrocarbon gases, which are uncondensable at normal atmospheric pressure and ambient temperature        iii. Solid residual char, which comprises carbon (coke) from thermal degradation of polymer along with any feedstock contaminants, such as, mud, dirt, sand, sludge, additives etc.        
Thermal cracking of waste plastic feedstock in the absence of catalyst requires higher energy input and often yields low economic value mixtures of hydrocarbons (mostly waxes), which having very broad compositional range, sometimes extending from light alkane gases to heavier solid hydrocarbons in the range upto C80. The product stream coming from thermal cracking of plastics needs intensive post-processing for it to be useful and of high economic value.
To solve the challenge of degradation of plastic wastes to generate useful lighter fuels, it is therefore necessary to establish optimal pyrolytic process conditions and catalyst(s), so as to obtain petroleum fuels directly from the process, which can be used as commodity fuels with reasonable amount of post-processing and refining. Compared to thermal degradation, thermocatalytic degradation using optimal reaction catalyst(s) and process conditions generates liquid fuels having a narrower carbon number distribution with a high percentage of the liquid fuel being in the C5-C24 range. When hydrocarbon-rich plastics such as HDPE, LDPE, PP, PS are used as feedstock, 4.0-7.0 barrels of light & middle petroleum distillates may be generated from one tonne of feedstock.
To enable thermocatalytic degradation, pulverized mixed waste plastics are pyrolized in an inert atmosphere in a pyrolysis reactor. The pyrolysis process results in formation of gaseous and non-gaseous byproducts. The catalyst may be added to the pyrolytic reactor itself (liquid phase reaction) as a certain percentage by weight of incoming waste plastic. In another method (vapor phase reaction), the gaseous byproduct coming from the pyrolysis reactor is reacted with a fixed amount of catalyst contained in a fixed, moving or fluidized catalytic reactor. The catalyst promotes the scissioning (cracking) of longer hydrocarbon chains in the evolved gaseous byproduct to form smaller length hydrocarbon chains, which are then condensed to obtain lighter fuel oils.
One of the challenges corresponding to pyrolysis is the removal of non-gaseous residual byproducts from the pyrolysis reactor. It shall be noted that if non-gaseous byproducts are not removed from the pyrolysis reactor, then the overall efficiency of pyrolytic degradation shall reduce, since such residual byproducts interfere with transfer of heat to incoming feedstock.
In one of the techniques, a mixture of plastics and tire scrap is fed to a batch pyrolytic reactor equipped with a special mixer, which agitates the feedstock during the pyrolysis process. In this semi-continuous system, raw materials are fed to the pyrolytic reactor for some time and the main byproducts are liquids and gases, which are collected separately. The residual matter, which is a mixture of carbon char, feedstock impurities and inorganic matter, remains in the pyrolytic reactor. At the end of the production cycle, the pyrolytic process is stopped and the rotational direction of agitating mixer is reversed, such that the mixer arms now scrape residual material from the reactor walls. The main disadvantage of this arrangement is a semi-continuous working mode and relatively low output from the production plant.
In U.S. Pat. No. 6,866,830, a fluidized-bed is used to pyrolytically treat mixed waste plastics. While fluidized-bed reactor has numerous advantages, such as, improved heat transfer to the plastic, continuous dosing of catalyst (through an online regenerator) and continuous removal of coke deposited on the catalyst, nevertheless, fluidized-bed pyrolysis of plastics has problem related to stickiness of the sand particles (the fluidization medium) that becomes coated with fused plastic. This may require a continuous withdrawal of bed material that must be substituted with a make-up of fresh material.
In Canadian patent 1127575, a horizontal pyrolytic reactor is used. The reactor receives feedstock at a first end, and the gaseous and non-gaseous byproducts of pyrolysis are removed from a second end. The movement of the feedstock and the byproducts from the first end to the second end is enabled by a screw conveyer that extends along the length of the reactor. Usage of such a screw conveyer decreases the volumetric space available for feedstock in the reactor. Further, byproducts might stick to the surface of the screw conveyers and lead to undesirable effects.
In various methods related to conversion of plastic waste to useful fuels, it has been studied that, to remove non-gaseous residue from the pyrolysis reactor, the pyrolytic apparatus often includes elaborate mechanisms such as irregular shaped metal scrapers, which scrape the inside walls of the pyrolytic reactor. Such scrapers typically consist of an assembly of shafts and blades. These shafts and blades take away valuable volumetric space which would have otherwise been available to the feedstock, thereby leading to an increase in the size of the pyrolytic reactor. Further, the scraping assembly to be effective, needs to maintain precisely the requisite gap from the pyrolysis reactor side-walls. This makes the efficacy of the scraping assembly vulnerable to mechanical fatigue, wear and tear, vibrational and other forces during the pyrolytic process. Further, the pyrolysis process has to be stopped from time-to-time for removal of such accumulated non-gaseous residue from the pyrolysis reactor leading to loss of energy and process productivity. U.S. Pat. No. 6,777,581 for example, uses a pyrolytic chamber with an auger positioned at its base for scraping the residual material and carbon char deposits. Char removal is only activated when the vessel is cooled.
Hence, in light of the foregoing discussion, there is a need for technique which provides a mechanism for pyrolytic conversion of waste plastics into liquid fuel oils and which includes a cost-effective and robust mechanism for continuous removal of non-gaseous residual matter from the pyrolysis reactor without the need to halt the process.